1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device.
2. Description of Related Art
Active matrix liquid crystal display devices are known as being effective for high-quality display. They are constructed such that thin-film transistors are formed on a transparent substrate (usually a glass or quartz substrate) for respective pixels. Each thin-film transistor controls charge that enters or exits from an electrode (pixel electrode) of the associated pixel. The active matrix liquid crystal display devices require circuits (peripheral circuits) for driving the thin-film transistors for the respective pixels. In general, the peripheral circuits are constructed as an external IC circuit called a driver IC.
In an advanced version, the peripheral circuits formed by using thin-film transistors are integrated on the substrate. Providing a unified structure in which the pixel region and the peripheral circuit regions are integrated on the same substrate, this configuration facilitates the use of a liquid crystal panel.
As an example of application of the above liquid crystal panel, a projection-type liquid crystal display apparatus will be described below.
A first method of performing color display is to form color filters of R (red), G (green), and B (blue) in a liquid crystal panel. A second method is to prepare a plurality of panels and combine images formed by those panels. In recent years, with an increasing need for large screen display, the second method is used more frequently to implement a projection-type display apparatus, because in the first method the substrate size needs to be increased and hence it is difficult to manufacture a panel. The second method is disclosed in Japanese Utility Model Laid-Open No. 58-111580.
In the second method, to combine images, the consistency of optical axes is important. Conventionally, liquid crystal panels are arranged independently and the modulating of optical axes is performed by adjusting the position and orientation of each panel in a subtle manner. However, this, is not preferable because it causes a cost increase and complicates the structure of the apparatus. There is known a further technique in which the same images are superimposed on each other to increase the screen size or the brightness. However, this technique has a problem of cost increase because it complicates the apparatus structure.
To solve the above problems, attempts have been made to integrate the three panels into a single panel. In this case, it is basically sufficient to generate a set of images corresponding to three colors of R, G and B. The brightness can be increased by generating two or more sets of images corresponding to R, G and B.
In this type of configuration, in forming peripheral driver circuit regions, it has been attempted to locate peripheral circuits that should be integrated at a high density at positions as close to the center of a substrate as possible, to increase a final production yield.
However, the above conventional liquid crystal display devices have two problems described below.
The first problem is as follows. A black matrix which is made of a reflective metal such as Cr and occupies a large area of a display screen is formed on the inside surface of a upper transparent glass substrate that is located on the display screen side. External light is reflected by the black matrix and comes out of the display screen. This lowers the contrast of a displayed image and hence makes it less visible, that is, lowers the display quality.
The second problem relates to a case where a black matrix is formed on an opposed substrate. In this case, as shown in FIG. 11A, a black matrix 1 is so formed as to overlap with ITO pixel electrodes 2 by 5-7 μm in consideration of the bonding accuracy of the TFT substrate and the opposed substrate. Thus, the size of opening portions is restricted. In this case, to increase the brightness of the display device, it is necessary to employ a brighter back light, resulting in an increase in power consumption.
FIG. 11A shows how the black matrix 1 on the opposed substrate and the ITO pixel electrodes 2 overlap with each other. Reference numerals 3-5 denote a signal line, a TFT, and a scanning line, respectively.